The Open System Interconnection (OSI) standard established by the International Standards Organization (ISO) provides a seven-layered hierarchy between an end user down to a physical device through which different systems can communicate. Each of these seven layers performs different tasks and they are specified by the OSI standard for their inter-layer as well as inter-device interaction.
The physical layer defines the physical and electrical characteristics of the network. It provides the hardware means of sending and receiving data on a carrier. The data link layer defines the access strategy for sharing the physical medium. It also describes the representation of bits on the physical medium as well as defining the format of messages on the medium. In addition, it is also responsible for proper synchronization of data sending in blocks (or frames). The network layer provides a means for communicating open systems to establish, maintain and terminate network. In addition, it also handles routing and forwarding of data to proper destinations. The transport layer manages the end-to-end control and error checking to ensure complete data transfer, data reliability and integrity. The session layer provides for two communicating presentation entities to exchange data with each other. The presentation layer is where application data is packed/unpacked, ready for use by the running application. Protocol conversions, encryption/decryption and graphics expansion also takes place in this layer. The application layer is where end-user and end application protocols such as telnet, ftp and mail (pop3 and smtp) are found. Here, communication partners, quality of service and any constraints on data syntax are identified, and user authentication and privacy are considered.
The Institute of Electrical and Electronics Engineers (IEEE) 802 LAN/MAN committee, which develops Local Area Network (LAN) and Metropolitan Network (MAN) standards, has developed a three-layered architecture that corresponds to the physical layer and data link layer of the OSI standard.
The IEEE 802 standard includes a physical layer (PHY), which essentially has the same role and functionalities as the physical layer of the OSI standard. The data link layer of the OSI standard is broken down into media access control (MAC) layer and logical link control (LLC) layer. Together, the MAC layer and LLC layer shares the same functionality of the data link layer of the OSI standard. Individually, the LLC layer places data into blocks/frames that can be communicated at the PHY layer whereas the MAC layer manages communication over the data link, data transmission and reception, as well as data acknowledgement (ACK) frames.
FIG. 1 shows the block diagram of two wireless networks, for example (100) and (102), each is wireless personal area network (WPAN) that could use the IEEE 802 standard. Both networks apply to network settings whereby bandwidth is to be shared among several users, such as wireless local area networks (WLAN), or any other appropriate wireless network.
In FIG. 1, the first network (100), for example a WPAN, consists of a central controller (104) of the first network (100), as well as dependent devices, for example (106), (108), (110), (112), that connect to the central controller (104). The radio range (114) of the central controller (104) determines the overall coverage of the first network (100). Each device, for example (106), (108), (110), (112) in the first network (100) is able to communicate via unidirectional/bi-directional radio links, for example (116), (118), (120), (122) with the central controller (104). Devices connected to the first network (100) may establish direct peer-to-peer unidirectional/bi-directional links, for example (124), (126) with each other for communication, depending on their radio range or security issues. The central controller (104) controls the time allocation and the media access of the first network (100).
Typical wireless networks like the ones illustrated in FIG. 1, that each has a central controller to which other dependent devices connects to, use a MAC layer based on centralized control. One such example of centralized MAC is the IEEE 802.15.3 High Speed WPAN standard. Centralized control schemes for MAC layer have the key advantage of predictability of data throughput performance. This is due to the fact that the central controller allocates and manages the media usage of all devices in the network. However, centralized control schemes pose problems as described below.
The main problem of a centralized control is the dependency of all devices on a central entity for timing allocation. When the central control device goes out of range or switched off, the wireless network collapses unless another device takes over the role of the central controller. Such situations may occur frequently especially in ad hoc networks where devices are highly mobile. Thus, a centralized control based MAC layer is not suitable for ad hoc networks where the devices are highly mobile.
A second problem with a centralized control scheme is that devices in such a network are not guaranteed to be able to communicate with peer devices also in the same network, due to limited radio range. Being in a centralized network only ensures that each device is able to communicate with the central controller. With reference to FIG. 1, device 5 (112) is in the WPAN (100). To device 5 (112), device 1 (104), 2 (106), 3 (108) and 4 (110) are also in the same WPAN (100). Yet, device 5 (112) can only communicate with device 1 (104) and not the rest of the members of the WPAN (100) due to its range limitation.
A third problem with a centralized control scheme is illustrated using FIG. 1. Here, the network environment (128) consists of, for example two WPANs (100) and (102) respectively, centred around controllers device 1 (104) and device 6 (130) respectively. Due to the fact that device 4 (110) and device 7 (132) belong to different WPANs, ie. (100) and (102) respectively, they are not able to communicate although they are positionally close to each other. From an end user point of view, for the case of an ad hoc device, this is an inconvenience issue. For example, a typical such scenario will be when a Personal Digital Assistant (PDA) device placed by a user onto a desk in the office near a notebook PC and digital camera only for the user to find that the PDA can only communicate with the digital camera because the notebook PC is a member of a separate WPAN centred around another wireless device further away.
A fourth problem associated with a network based around a central controller is the issue of interference from neighbour networks. With reference to FIG. 1, device 4 (110) is out of range of device 6 (130), which is the central controller of WPAN (102). Likewise for device 7 (132), it is out of range of device 1 (104), which is the central controller of WPAN (100). This means that device 1 (104) and device 6 (130) has no idea of the existence of device 7 (132) and device 4 (110) respectively as well as their relative close proximity, due to both belonging to different networks, for example (100) and (102). Therefore, interference may occur when device 1 (104) or device 3 (108) sends data to device 4 (110) and simultaneously, when device 7 (132) sends data to device 6 (130). Since device 7 (132) is relatively close to device 4 (110), interference may be experienced at the receiving device 4 (110).